It is common practice to fabricate microwave strip line assemblies by patterning a conductor on a laminate. The pattern is capped with a second laminate in a bonded assembly. The outer surfaces of the assembly can then be clad with a conducting material to form two ground planes. Shielding vias may be used to connect the ground planes. Multi-layer circuits of this form can be produced and this type of bonded assembly is relatively easy and inexpensive to fabricate and is also robust.
A disadvantage associated with such assemblies when higher performance is required is that they exhibit relatively high microwave loss. A recently introduced alternative to these assemblies is a suspended substrate stripline (SSS) structure, where the conductor is patterned on a thin dielectric that is suspended between the two ground planes. Thus, the volume between the ground planes can mainly include air, which results in lower levels of microwave loss. A rigid silicon-based material is normally used for supporting the conductor. However, these structures are more expensive to produce than the bonded assemblies and problems can arise because silicon absorbs water and has different characteristics, e.g. dielectric constant, to the other materials that are commonly used in the circuit assembly.